1. Field of the Invention
The present invention relates to a recording liquid composition generally called ink for use in the so-called ink jet recording system.
2. Description of the Prior Art
Among various recording systems hitherto known in the art, non-impact recording system is comparable to impact recording system in simplicity. Moreover, the non-impact recording system has a particular advantage that the noise generated during recording is negligibly small. For this reason, all interests in the art are being focused on the non-impact recording system. Among others, the ink jet recording system has been found to be the most advantageous nonimpact recording system. It allows an especially high speed recording and also it enables to effect recording on a common paper sheet without any need of special fixing process.
Various improvements in the ink jet recording system have been proposed until now. Some of them have already been introduced successfully in the market and some are still under development.
In brief, the ink jet recording system is a recording system in which droplets of ink are jetted from a small orifice toward a recording material such as a paper sheet in a controlled manner and the jetted droplets of ink are adhered onto the recording material so as to record a desired image on it. In view of differences in the method of generating ink droplet, the method of controlling the flying direction of the ink droplet and the like, the ink jet recording system is classified into several types. Typical one is shown in FIG. 1 of the accompanying drawing. This system is of the type in which a recording signal is applied to a recording head part containing a piezo-oscillator and droplets of the recording liquid are generated to effect recording in response to the signal.
In FIG. 1, a recording head is designated by 1, a piezo-oscillator by 2a, an oscillating plate by 2b, an inlet of ink by 3, a liquid chamber in the head by 4 and a jet orifice by 5. Recording liquid 7 is introduced into the liquid chamber 4 from a storage tank 6 through a supply tube 8. In case of necessity, intermediate processing means 9 such as pump or filter may be provided in the supply tube 8. Recording signal S is converted into a pulse by signal processing means 10 such as a pulse converter and then applied to the piezo-oscillator 2a. Change in pressure on the recording liquid within the liquid chamber 4 caused by the signal results in droplets 11 of the recording liquid being jetted through the jet orifice 5. Thus, an image is printed on the surface of a recording material 12 with the droplets.
Another type of ink jet recording system is shown in FIG. 2 which comprises an liquid chamber 4 so shaped as to form a nozzle. A cylindrical piezo-oscillator 2a is provided surrounding the chamber 4. Mechanism for producing droplets of recording liquid in this apparatus is essentially the same as that in the apparatus shown in FIG. 1.
Although not shown in the drawing, there are also known those types of apparatus in which droplets of the recording liquid are continuously produced while electrifying the droplets to use a part of the droplets for recording or in which signals in a form of thermal energy are applied to the recording liquid in an liquid chamber provided within a recording head.
As previously described, according to the ink jet recording method, droplets of recording liquid composition, that is ink are jetted toward a recording material on which an image is to be recorded with the ink droplets. The ink used for this purpose is a composition composed essentially of a recording agent which is generally dye or pigment, and a liquid medium or carrier in which the recording agent is dissolved or dispersed. As the carrier, there is usually employed water or any suitable organic solvent or a mixture thereof. Various additives may be added to the composition as required.
As described above there are known various types of ink jet recording systems. One of the most important things common to all of the types is that the ink composition should have well-controlled values of physical properties such as viscosity, surface tension and specific resistance suitable for the droplet generating method and the direction controlling method of flying droplet then used. Another important requirement is that the ink composition should never form any solid matter even when some component of the recording composition is vaporized or the chemical composition of the ink is changed during a long storage period or during a rest time of recording operation. Since the jet orifice of the ink jet recording apparatus is extremely small (generally in the order of 10.mu. in diameter), if solid is formed in the liquid ink composition, then it will lead to blocking of the orifice. In the worst case, the orifice becomes unable to jet ink droplet at all.
Even if such worst case is seldom, solids formed in the ink composition will inevitably disturb the uniformity of droplets and the stable flying of droplets, which will in turn reduce the recording ability, jet stability, jet responsiveness and continuous recording ability. Change in chemical composition of the ink brings forth change in physical characteristic values of the ink once adjusted to the desired values at the time of formulation. This may also have adverse effect on the recording ability, jet stability and jet responsiveness. Under the standard of the prior art there is an eager demand for such liquid carrier which is hardly evaporated and such recording agent which has a high chemical stability or such combination of recording agent and carrier which eliminates the above described disadvantages.
It is also required that the recorded image should have sufficiently high contrast and sharpness. In general, the known recording ink is apt to cause a trouble of blockage of the jet orifice when the concentration of the recording agent in the ink composition is increased to improve the contrast of the recorded image. Therefore, at present, such recording agent is being wanted which has a high solubility in liquid carrier as well as a high sharpness in tint.
Furthermore, the image once recorded has to have good water resistance, light resistance, weather resistance and abrasion resistance. However, the ink compositions, especially aqueous ones according to the prior art have the drawback that they easily get blurred on the recording material when wetted or they become faded by light or mechanical friction. Therefore, those skilled in the art have been longing to provide improved ink composition free of these drawbacks.
Many attempts have been made to improve the ink composition in the above mentioned respects. However, until now there has not yet been obtained such ink composition which can satisfy all of the requirements mentioned above and which is practically useful.